C. JOHANSEN and P.C. KERRIDGE

CSIRO Division of Tropical Crops and Pastures, Cunningham Laboratory, Cnr. Carmody Road and Chancellors' Place, St. Lucia, Qld, Australia 4067.

Abstract

Relations between dry matter yield and N concentration in shoot samples were derived for tropical legumes responding to Mo application in field experiments at different sites. Critical N concentrations, those at 90% of the fitted maximum yield, varied widely (2.2 to 3.6% N). There were no consistent differences between harvest times, sites or species. The wide variation in N response curves obtained prevented the establishment of critical N concentrations sufficiently precise to indicate whether growth of Siratro, desmodium, glycine or lotononis was being limited by N supply. Only extreme cases of deficiency (< 2% N) and sufficiency (> 3.6% N) could be reliably defined.